Many improvements have been made in oil-drilling technology in the past two decades. As a result, operators are able to drill faster, deeper, and longer. Along with these capabilities, however, have come greater costs and greater needs for efficiency.
Oil exploration is performed by drilling test holes using an oil-drilling bit connected at the lower end of a drill string. Technological advances now allow advanced directional and horizontal drilling, using steerable drilling systems. However, the ability to steer an oil-drilling bit necessitates that the desired path of the drilling be continually reevaluated. Other drilling parameters, dictated by considerations of varying complexity, must also be specified as drilling proceeds. Many decisions affecting drilling operations are based on physical properties of formations within the bore hole. Knowledge of these physical properties is also used to determine reservoir properties and capacities. In short, an accurate knowledge of bore hole conditions allows an operator to determine whether he is drilling in the right place. The sooner the operator receives this knowledge, the more efficient will be his decisions regarding drilling activities.
When searching for hydrocarbon-bearing sub-surface earth formations, many drilling path decisions and other decisions relating to drilling activities are most desirably based on the actual hydrocarbon content of encountered sub-surface formations. At one time, the primary technique for determining hydrocarbon content was to analyze actual core samples taken from the sides of the bore hole itself after drilling. Hydrocarbon measurements, made directly upon these samples, resulted in direct and seemingly highly reliable data.
This technique, although still in use, has been found to be less accurate under some conditions than had previously been thought. For instance, by the time core samples are taken, hydrocarbon-bearing fluids initially contained therein may have been lost or diluted. In addition, core samples are often not representative of the sub-surface rock formation as a whole.
More recently, so called "wire" or "wireline" logging has been used to make indirect measurements of physical properties in a bore hole. Wireline logging involves withdrawing the drill string and then lowering a data-logging instrument by wire or cable into the open bore hole. A wireline logging instrument performs various measurements as a function of depth, including, for example, bore hole diameter, electrical potential between formation beds, radioactive impurities in the formation, and electrical resistivity. Hydrocarbon concentrations are inferred from this information.
The primary disadvantage of wireline logging is that, to date, these instruments are incapable of making direct hydrocarbon measurements. Rather, hydrocarbon content must be inferred from the measurements noted above. A further disadvantage is that measurements may be affected by drilling fluids which fill the bore hole.
Both of the above methods suffer from the additional disadvantage that measurement results are delayed for a significant time after the drilling bit has encountered specific formations. In addition, the measurements are performed only at discrete, and rather lengthy, time intervals, and require drill string withdrawal. The delayed and discrete nature of the measurements limits an operator's ability to respond quickly to changing bore hole conditions, since many such conditions will become known to the operator, if at all, only after the drilling bit has passed the point where the conditions were encountered.
Another notable technique for determining in-hole hydrocarbon concentrations is that of "mud logging." In mud logging, drilling fluid, often called drilling mud, is analyzed for hydrocarbon content. Drilling mud is a water-based fluid which is pumped through the drill string to the drill bit during drilling. The mud then rises upward around the drill string to carry cuttings and fluids encountered by the drilling bit to the surface. Mud samples are collected as the mud reaches the top of the bore hole. Gases are withdrawn from the mud samples and analyzed by a conventional gas chromatograph technique for presence of hydrocarbons. The hydrocarbon content of the mud reaching the surface is correlated to the depth at which the mud emerged from the drill string to determine hydrocarbon content at that depth.
While mud logging provides accurate measurements of hydrocarbons occurring in the drilling mud, these measurements do not always correspond to the actual hydrocarbon content of encountered sub-surface formations. For one thing, drilling mud itself, as injected into the bore hold, often contains hydrocarbons such as diesel oil which must be accounted for. In addition, as the drilling mud rises through the bore hole it accumulates hydrocarbons from previously-encountered sections of the bore hole. It is not always possible to predict or account for this accumulation in relation to the hydrocarbons originating from the bottom of the bore hole. Furthermore, concentrations of hydrocarbons actually originating from the bottom of the bore hole are often dispersed and diluted by the time the drilling mud reaches the surface.
Another problem with mud logging is that measurements are often delayed more than thirty minutes from the time the drilling bit actually encounters a specific hydrocarbon formation. This is the time required for the drilling mud to rise around the drill string to the surface. Additional time is often consumed in de-gassing and performing actual measurements on the drilling mud. While the delay is not as serious as with the previously-mentioned techniques, it is still of concern to an operator faced with critical decisions regarding drilling paths and other parameters.
Measurement While Drilling (MWD) technology provides real-time information regarding bore hole physical properties near the drilling bit during actual drilling. The first commercially feasible MWD sensors reached the market in the early 1970's, and were concerned primarily with drill guidance, where real-time information is imperative. Since then, however, MWD sensors have been developed for making a variety of formation measurements.
MWD instruments are located within specially designed drill collars near the drill bit at the base of the drill string. The instruments take various samples during the drilling process, and the information gathered may be stored in downhole memories and/or transmitted to the surface via acoustic pulses in the drilling mud. Power is provided either by internal batteries or by power turbines driven by flowing drilling mud.
To date, MWD instruments provide geological data on downhole formations much like wireline logging, except that the data is provided on a continuous basis in real time. This data may be used both to enhance overall drilling efficiency and to evaluate hydrocarbon production potential of the formations encountered. Resistivity and gamma measurements, from which hydrocarbon concentrations can be inferred, are the primary means of evaluating drilling efficiency when using MWD systems.
At this time, however, there are no commercially available MWD probes for making direct and continuous measurements of hydrocarbon concentrations at the base of the bore hole. One reason for the absence of such a sensor is the extremely harsh environment in which MWD instruments must operate, resulting primarily from the fact that the sensors have to be placed in a drilling bottom-hole assembly. The instruments must therefore provide measurements in an environment that is hostile from the standpoint of pressure, vibration, shock, flow, and the presence of cuttings. Additionally, all of the instruments must be housed in a small space while not impairing the mechanical integrity of the drill string assembly.
In spite of the harsh environment, reliability of MWD sensors is a critical issue. The cost of prematurely withdrawing a drill string to repair or replace a malfunctioning MWD sensor is very high.
Accordingly, there is a definite need for a device which provides continuous and real time hydrocarbon measurements at the base of a bore hole during drilling.